LAMELLIBRANCHIATA 



505 



sions. Even in the Protobranchiata, the cihary apparatus for food- 

 collecting has been developed as in the rest of the group, and it has 

 been pointed out that there are ciliated discs, adjacent pairs of which 

 act as ciliary junctions and hold the filaments together to form 

 lamellae. There is, moreover, a subdivision of the mantle cavity into 

 inhalant (ventral) and exhalant (dorsal) chambers in spite of the small 

 size of the ctenidia. 



The blood system of the lamellibranchs is best explained by refer- 

 ence to that of Mytilus, the common mussel (Fig. 371). Here the 



p. a 



Plicate 

 canals 



Fig. 371. Diagram of the circulation in Mytilus to show the greater import- 

 ance of the part of the system in the mantle and plicate canals. Of the blood re- 

 turned from the viscera a much smaller proportion is sent through the ctenidia. 

 Slightly altered from Field, Au. auricle; bl. bladder of kidney opening into 

 the pericardium; aff.c.v. afferent, ejf.c.v. efferent ctenidial vein; l.v. longi- 

 tudinal vein of kidney ; ^.ar. pallial artery; pc. pericardium; v. ventricle with 

 rectum, represented by a dotted line, passing through it. 



heart, as in Anodonta, consists of a ventricle surrounding the rectum 

 and two auricles, each of which opens into the ventricle by a narrow 

 canal and is attached by a broad base to the wall of the pericardium 

 over the insertion of the ctenidia into the mantle. A single vessel, the 

 anterior aorta (a posterior aorta is also present in Anodonta), leaves 

 the ventricle, dilates into an aortic bulb and then divides into many 

 arteries. Of these, the most important are the pallial arteries going to 

 the mantle and the arteries forming part of the visceral circulation 

 (the gastrointestinal, hepatic and terminal arteries, the last named 



